Source: http://www.mehrdust.com/archives/how-to-list-all-dnd-enable-extensions

in linux prompt run:

# asterisk -rx “database show DND”

# asterisk -rx “database show” | grep “/DND/”

# asterisk -rx "database show DND"
/DND/1002                                         : YES
/DND/1004                                         : YES
/DND/1006                                         : YES
/DND/1007                                         : YES

To remove DND from an extension run:

# asterisk -rx “database del DND ext.”

To enable DND manully for an extension run:

# asterisk -rx “database put DND ext. YES”

# asterisk -rx "database put DND 1002 YES"
Updated database successfully

# asterisk -rx "database show DND" /DND/1002                                         : YES